Hydraulic test

Hi,

We are currently converting IDEAS to the annex60 baseClasses. Using one of our examples, I discovered some problems with the DynamicPipe or with pipes in general based on the fixedResistanceDpM connected to an Mixing Volume with a heat port:

I coupled a step input to a prescribedTemperature which I connected to the dynamic pipe. Dymola compiles but does not simulate (fail to reduce DAE index, see: Annex60.HydraulicTest.PumpWithPipeHeatPort_failToReduceDAEIndex). This is due to the singularity of the time derivative of the step function which gives problems for the dynamics of the pipe.

If I firstly couple a firstOrder filter to the step function, then the problem is fixed (see Annex60.HydraulicTest.PumpWithPipeHeatPort).

I used then the same system Annex60.HydraulicTest.PumpWithPipeHeatPort but once with Annex60.Media.Water and once with Modelica.Media.Water.ConstantProperties. The temperatures are quite similar for this case, but the CPUs differ quite a lot:

![image](https://cloud.githubusercontent.com/assets/2987659/2590886/18fdc318-ba64-11e3-86f2-3d921dfb3514.png)